High-nitrogen stainless-steel pipe with high strength high ductility, and excellent corrosion and heat resistance

ABSTRACT

Nitrogen (N) absorption and diffusion treatments are performed for the inner and/or outer surfaces of austenite stainless steel pipe materials in N gas atmosphere at temperatures near 1,100° C. to obtain nitrided stainless steel pipe materials having 0.25˜1.7% (mass) of solid solution nitrogen (N) including a gradient structure formed within the pipe wall in which the concentration of solid solution N continuously decreases gradually from the surface. The solid solution N present in the gradient structure promotes short range ordering (SRO) of substitutional alloying elements leading to homogenization of distribution of alloying elements in the austenite phase, generating an extremely high proof strength (yield strength) about 3 times as high as that of conventional austenite stainless steel pipe materials and enhancing characteristic of anti-hydrogen gas embrittlement (anti-HGE) so as to be suitable for use in a high pressure hydrogen tank utilized in hydrogen cell vehicle (FCV) and a liquid hydrogen tank.

RELATED APPLICATIONS

The present application is National Phase of International ApplicationNo. PCT/JP2011/052962 filed Feb. 4, 2011, and claims priority fromJapanese Application No. 2010-023208, filed Feb. 4, 2010.

ART FIELD

The present invention relates generally to a stainless steel pipe, andmore particularly to a high-nitrogen stainless steel pipe and hollowmaterials of various shapes and sizes, which are formed from the steelpipe, with a high strength and ductility, and an excellent corrosion andheat resistance, and process for producing same.

BACKGROUND OF THE INVENTION

An austenite stainless steel with excellent corrosion and heatresistance is utilized as important pipe arrangement materials in a widerange of industrial field such as steam-power generation, atomic powergeneration, automotive engineering, petrochemistry and, chemicalengineering.

Recently, austenite stainless steel pipes with high nickel content arebeing noted especially as compressed hydrogen gas storage tank materialsto be employed in a fuel cell vehicle (FCV) that is regarded as the mostlikely candidate car, among eco-friendly cars, immediately following anelectric vehicle (EV) which has already been put to practical use.

The reason why austenite stainless steel pipes with high nickelconcentration are noted as the aforementioned tank materials is due toenduring hydrogen gas embrittlement (HGE) under compressed hydrogen gasenvironment, while most metal pipe materials tend to cause HGE in suchhydrogen gas environment.

However, application of conventional austenite stainless steels havinghigh nickel concentration such as SUS316L and SUS310S to the compressedhydrogen gas storage tank material will be impossible, because of theirlow strength.

Therefore, such high nickel austenite stainless steel pipes have to belargely strengthened without loss of their ductility in order to employthem as the hydrogen tank.

As nitrogen (N) in an amount of, e.g., about 0.9% (by mass) is added toa chromium-nickel type stainless steel having a composition equivalentto that of SUS316L which is typical high nickel austenite stainlesssteel, the resulting stainless steel increases in offset yield strength(yield strength) to about three times as high as that of SUS316Lstainless steel, with no decrease in fracture toughness yet with muchmore improvements in corrosion resistance, especially in pittingcorrosion resistance.

So far, high-N austenite steels having nitrogen in an amount of up toabout 0.1 to 2% (by mass) have been produced by melting-solidificationprocesses usually in nitrogenous atmospheres.

In such melting solidification processes, however, a large amount ofnitrogen gas liberates during solidification of the liquid phase due tothe nitrogen solubility gap between both phases of liquid and solid,leading to formation of faults like blow holes in the solidifiedproducts. Moreover, there are difficulties that segregation generatingin the solid phase cannot be avoided especially in large solid products.Accordingly, sound products without such faults are difficult to obtainby the melting-solidification processes as mentioned above.

Now, a high-nitrogen austenite stainless steel material, too, has beenintensively tried to manufacture by a nitrogen-absorption and soliddiffusion process (also called a solution nitriding process) wherein anaustenite stainless steel is treated in nitrogen gas atmosphere in arange of temperatures as high as 1200° C. to cause nitrogen (N) to beabsorbed into the surface of the steel and diffused into the solidphase.

However, since such N absorption and diffusion processing is usuallyperformed in relatively high temperature regions of 1200° C. or above,it causes enlargement of the crystal grain in the austenite steelmaterial, resulting in a marked loss of its ductility in contrast to ahighly increase in strength thereof due to forming a high concentrationsolid solution of N.

In addition, it is practically difficult to effectively apply the Nabsorption and diffusion process to steel plate or pipe materials with arelatively large wall thickness in view of the time required in suchprocess.

Especially, in the case where N absorption and diffusion treated pipematerials are utilized, for instance, as hollow materials like fuel gasstorage tank, for fuel cell vehicles (FCVs), with relatively largedimensions, e.g. diameter and thickness, they are required to have notonly their own ductility but also strength sufficient thereto; anysatisfactory austenite stainless steel material is not achievable asyet.

DISCLOSURE OF THE INVENTION

The present invention has for its objection the provision ofsatisfactory solutions to the above problems.

Basically, the present invention makes use of nitrogen absorption anddiffusion processing for an austenite stainless steel pipe material,wherein the stainless steel pipe materials are treated in nitrogen gasatmosphere in a range of temperatures near 1000 to 1100° C. not higherthan the critical temperature for crystal grain enlargement of the steelmaterial to cause nitrogen (N) to be absorbed into the surface of thesteel material and diffused into the solid phase. The resulting Nabsorption and diffusion processed stainless steel material has agradient structure composed of a part that is close to the steel surfacepart and has been highly strengthened by the formation of a highconcentration solid solution of N and a part in which ductilitygradually increases toward around the center of the cross-section of thesteel as the N concentration decreases, and the enlargement of crystalgrains is minimized during the treatment, thereby providing a nobelaustenite stainless steel pipe material with high strength and ductilityand excellent corrosion and heat resistance.

Furthermore, the thus-obtained stainless pipe material is strengthenedby slight plastic working so as to provide a nobel stainless steel pipematerial showing much better strength and ductility.

The present invention also provides a nobel process for manufacturing aN absorption and diffusion treated high-nitrogen austenite stainlesssteel pipe material with fine crystal grains leading to an improvedelongation (ductility) as well as a high strength by applying grainrefinement treatment utilizing, for instance, eutectoid transformationof the austenite phase to N absorption and diffusion treated steelmaterial.

Furthermore, the present invention provides a nobel process formanufacturing a high-nitrogen austenite pipe or hollow materials whosemanufacture is difficult to realize using a single N absorption anddiffusion processed stainless steel pipe material alone, in which aplurality of N absorption and diffusion processed austenite steel pipeof the same quality are disposed one over another so as to result indimensions, e.g., diameter and wall thickness, according to the use orstrength level, and the pipe arrangement is united by adhesionprocessing through hot drawing, hot rolling, or other methods, therebyobtaining a high-nitrogen austenitic steel pipe or hollow material whichhas high strength and ductility, and excellent corrosion and heatresistance and has repetitions of the gradient structure within theaustenitic steel wall.

The stainless-steel pipe or hollow material can have large or smallsizes and be of various kinds, and examples thereof include a hollowmaterial for use as container for storing high-pressure hydrogen gaswhich is for fuel cell vehicles (FCVs) and which does not sufferhydrogen gas embrittlement (HGE) in the presence of high-pressurehydrogen gas.

Thus, the present invention is concerned with high-nitrogen austenitestainless steel pipes and hollow materials constructed as recited below,and their manufacture processes and uses.

<1> A high nitrogen stainless steel pipe with a high strength andductility, and an excellent corrosion and heat resistance, comprising anaustenite steel containing 0.25 to 1.7% (by mass) of solid solutionnitrogen including a gradient structure formed within the pipe wall inwhich the concentration of solid solution nitrogen continuouslydecreases gradually from the surface, characterized in that the outsidesurface and/or the inside surface of said steel pipe are in contact witha substance that becomes a nitrogen source in a range of temperaturesnot higher than the critical temperature for crystal grain enlargementof the steel pipe material, so that nitrogen is absorbed into thesurface of the pipe and diffused into the solid phase, and formed aresaid austenite solid solution and said gradient structure whichcomprises a part that is close to the surface and has been highlystrengthened by the formation of a high concentration solid solution ofnitrogen and a part in which ductility gradually increases toward aroundthe center of the cross-section of the pipe as the nitrogenconcentration decreases, moreover subsequently annealing the pipe withsaid gradient structure in vacuum, inert gas such as argon gas or anatmosphere of a gas with some reducing substance such as H₂ gas addedthereto in said range of temperatures leads to a gradual decreasing ofthe concentration gradient of nitrogen in said gradient structure,

or furthermore the resultant pipe material is strengthened by plasticworking such as drawing, rolling, extrusion or the like.

<2> A high nitrogen stainless steel pipe with a high strength andductility, and an excellent corrosion and heat resistance, comprising anaustenite steel containing 0.25 to 1.7% (by mass) of solid solutionnitrogen including a gradient structure formed within the pipe wall inwhich the concentration of solid solution nitrogen continuouslydecreases gradually from the surface, characterized in that the outsidesurface and/or the inside surface of said steel pipe are in contact witha substance that becomes a nitrogen source in a range of temperatures(1) not higher than the critical temperature for crystal grainenlargement of the steel pipe material or in a range of temperatures (2)exceeding said critical temperature, so that nitrogen is absorbed intothe surface of the pipe and diffused into the solid phase, and formedare said austenite solid solution and said gradient structure whichcomprises a part that is close to the surface and has been highlystrengthened by the formation of a high concentration solid solution ofnitrogen and a part in which ductility gradually increases toward aroundthe center of the cross-section of the pipe as the nitrogenconcentration decreases, moreover subsequently annealing the pipe withsaid gradient structure in vacuum, inert gas such as argon gas or anatmosphere of a gas with some reducing substance such as H₂ gas addedthereto in said range of temperatures leads to a gradual decreasing ofthe concentration gradient of nitrogen in said gradient structure,furthermore, for thus obtained pipe material through said nitrogenabsorption-diffusion and annealing processing, the following crystalgrain refining (crystal grain size reducing) treatment (a) or (b) ispracticed:

(a) heating leading to austenitizing said steel pipe material (procedure<i>), subsequently slow-cooling such as air cooling (procedure <ii>)leading to decomposition into a mixture of fine ferrite and nitrideutilizing the eutectoid transformation of the austenite, and practicing1 or more repetitions of a series of these procedures wherein the finalcooling of procedure <ii> is rapidly continued to retain the refinedaustenite, or

(b) plastic working (procedure <i>) said steel pipe using drawing,rolling, extrusion or the like below the recrystallization temperatureof the steel pipe material, subsequently heating it to the austeniteregion, i.e., austenitizing (procedure <ii>), thereafter rapidly coolingsaid austenitized pipe material (procedure <iii>) to retain the refinedaustenite, and practicing 1 or more repetitions of a series of theseprocedures,

or moreover the resulting steel pipe material is strengthened by plasticworking such as drawing, rolling, extrusion or the like.

<3> A high nitrogen stainless steel pipe material with a high strengthand ductility, and an excellent corrosion and heat resistance,comprising plurality (1) of said stainless steel pipe, surface-polishedto generate its original surface, of any one according to said <1> to<2> above disposed one over another so as to result in dimensions, e.g.,diameter and wall thickness of the pipe, depending on the use orstrength level, or plurality (2) of said stainless steel pipe and otherpipe, surface-polished to generate its original surface, as adhesionmaterial disposed one over another as in the case of said plurality (1),wherein one or one or more selected from a group of said other pipedescribed below as (a) to (e) is sandwiched between said stainless pipe;(a) former austenite stainless steel pipe without nitrogen absorptionand diffusion plus annealing treatment, (b) other high nickel austenitestainless steel pipe, (c) nickel or nickel alloy pipe, (d) aluminum oraluminum alloy pipe, and (e) other pipe as adhesion material,characterized in that the pipe arrangement shown in said plurality (1)or (2) is united by adhesion processing through hot or warm plasticworking such as drawing, rolling, extrusion or the like, leading to ahigh nitrogen austenite pipe or hollow materials whose manufacture isdifficult to realize from a single nitrogen absorption and diffusionprocessed stainless steel pipe material alone,

or furthermore the resulting united steel pipe material is strengthenedby plastic working such as drawing, rolling, extrusion or the like.

<4> The high nitrogen stainless steel pipe material with a high strengthand ductility, and an excellent corrosion and heat resistance accordingto any one of <1> to <3> above, characterized in that said pipe materialcomprises a kind of steel selected from a group of (1) austenitestainless steel, (2) ferritic stainless steel, and (3) ferrite-austenitestainless steel.<5> A process for manufacturing a high nitrogen stainless steel pipematerial, characterized by involving steps of:

keeping the outside surface and/or the inside surface of an austenitestainless steel pipe material in contact with a substance that becomes anitrogen (N) source,

heating said steel pipe together with said nitrogen source substance ata temperature of 800 to 1100° C. in a range of temperatures not higherthan the critical temperature for crystal grain enlargement of the steelpipe material to cause nitrogen to be absorbed into the surface of thepipe and diffused into the steel solid phase, thereby obtaining anaustenite steel pipe material with high concentration of solid solutionnitrogen having a gradient structure formed within the steel pipe wallthat comprises a part which is close to the surface part of the pipe andhas been highly strengthened by the formation of a high-concentrationsolid-solution of N and a part in which ductility gradually increasestoward around the center of the cross-section of the pipe as the Nconcentration decreases, and

applying to said heat-treated pipe material annealing treatment in saidrange of temperatures in vacuum, inert gas such as argon gas or anatmosphere of a gas with some reducing substance like H₂ gas addedthereto, to result in a gradual decreasing of nitrogen concentrationgradient, or

applying to said pipe material plastic working operation such asdrawing, rolling, extrusion or the like, thereby obtaining a highnitrogen stainless steel pipe material with a high strength andductility and an excellent corrosion and heat resistance, whichcomprises an austenite steel containing 0.25 to 1.7% (by mass) ofsolid-solution nitrogen having a gradient structure in which theconcentration of solid solution nitrogen continuously decreasesgradually from the surface toward around the center of the cross-sectionof the pipe.

<6> A process for manufacturing a high N stainless steel pipe material,characterized by involving steps of:

keeping the outside surface and/or the inside surface of an austenitestainless steel pipe material in contact with a substance that becomes anitrogen (N) source,

heating said steel pipe together with said N source substance at atemperature of 800 to 1100° C. in a range of temperatures (1) not higherthan the critical temperature for crystal grain enlargement of the steelpipe material, or at a temperature above 1100° C. in a range oftemperatures (2) exceeding said critical temperature, to cause N to beabsorbed into the surface of the pipe and diffused into the steel solidphase, thereby obtaining an austenite steel pipe material with highconcentration of solid solution nitrogen having a gradient structureformed within the pipe wall that comprises a part which is close to thesurface and has been highly strengthened by the formation of a highconcentration solid solution of nitrogen and a part in which ductilitygradually increases toward around the center of the cross-section of thepipe as the N concentration decreases,

applying to said heat-treated pipe material annealing treatment in saidrange of temperatures not higher than said critical temperature invacuum, inert gas such as argon gas or an atmosphere of a gas with somereducing substance like H₂ gas added thereto, to result in a gradualdecreasing of the N concentration gradient, and

applying to said annealed pipe material crystal grain refining (i.e.,crystal grain size reducing) treatment by the following (a) or (b):

(a) austenitizing (heating) said steel pipe material (procedure <i>),subsequently slow-cooling such as air cooling (procedure <ii>) leadingto decomposition into a mixture of fine ferrite and nitride utilizingthe eutectoid transformation of the austenite, and practicing 1 or morerepetitions of a series of these procedures wherein the final cooling ofprocedure <ii> is rapidly continued to retain the refined austenite, or

(b) plastic working procedure <i> said steel pipe using drawing,rolling, extrusion or the like below the recrystallization temperatureof the steel pipe material, subsequently heating it to the austeniteregion, i.e., austenitizing (procedure <ii>), thereafter rapidly coolingsaid austenitized pipe material (procedure <iii>) to retain the refinedaustenite, and practicing 1 or more repetitions of a series of theseprocedures, or

applying to said pipe material plastic working operation such asdrawing, rolling, extrusion or the like, thereby obtaining a highstrength and ductility and an excellent corrosion and heat resistance,which comprises an austenite steel containing 0.25 to 1.7% (by mass) ofsolid-solution nitrogen having a gradient structure in which theconcentration of solid solution nitrogen continuously decreasesgradually from the surface toward around the center of the cross-sectionof the pipe.

<7> A process for manufacturing a high N stainless steel pipe material,characterized by involving steps of:

disposing plurality (1) of said stainless steel pipe of any oneaccording to said <5> to <6> above one over another so as to result indimensions, e.g., diameter and wall thickness of the pipe, according tothe use or strength level or plurality (2) of said stainless steel pipeof any one according to said <5> to <6> above and other pipe as adhesionmaterial one over another as in the case of said plurality (1) whereinone or one or more selected from a group of said other pipe describedbelow as (a) to (e) is sandwiched between said stainless steel pipe; (a)former austenite stainless steel pipe without nitrogen absorption anddiffusion plus annealing treatment, (b) other high nickel austenitestainless steel pipe, (c) nickel or nickel alloy pipe, (d) aluminum oraluminum alloy pipe, and (e) other pipe as adhesion material,

applying to said plurality (1) or (2) of the pipe material adhesionprocessing like hot or warm plastic working such as drawing, rolling,extrusion or the like to cause to unite said plurality (1) or (2) in arange of temperatures 15 to 40% lower than the melting temperature orthe melting point in Kelvin scale of the adhesion material in anoxidation-inhibition atmosphere such as in H₂ gas or AX gas (75 vol % H₂gas+25 vol % N₂ gas), or

furthermore applying to said united pipe material plastic workingoperation such as drawing, rolling, extrusion or the like, therebyobtaining a high nitrogen stainless steel pipe material with a highstrength and ductility and an excellent corrosion and heat resistance,which basically comprises an austenite steel containing 0.25 to 1.7% (bymass) of solid solution nitrogen having a gradient structure in whichthe concentration of solid solution nitrogen continuously decreasesgradually from the surface toward around the center of the cross-sectionof the pipe.

<8> The process for manufacturing a high nitrogen stainless steel pipeaccording to any one of <5> to <7> above, characterized in that in thecase where NH₃ gas or NH₃ gas with some H₂ gas or argon gas is utilizedas a substance that becomes a nitrogen source wherein the nitrogenabsorption and diffusion treatment using said nitrogen source like NH₃gas substantially entails annealing processing, annealing after nitrogenabsorption and diffusion treatment according to any one of <5> to <7>above can be omitted.<9>

The process for manufacturing a high nitrogen stainless steel pipeaccording to any one of <5> to <8> above, characterized in that in saidmanufacturing process said steel pipe contains: (1) 0.02 to 0.10% (bymass) of aluminum (Al) or titanium (Ti) or (2) 0.03 to 0.15% (by mass)of aluminum and titanium in all as a prohibition element of crystalgrain coarsening during said heating process of N absorption anddiffusion, annealing, or adhesion processing of plurality of the pipematerial.

<10>

The process for manufacturing a high nitrogen stainless steel pipeaccording to any one of <5> to <9> above, characterized in that saidsubstance that becomes a nitrogen source is one or two or moresubstances selected from the group consisting of (1) N₂ gas, (2) NH₃gas, (3) N₂ gas or NH₃ gas with some H₂ gas or argon gas, (4) ironnitride, (5) chromium nitride and (6) manganese nitride.

<11>

The process for manufacturing a high nitrogen stainless steel pipeaccording to any one of <5> to <10> above, characterized in that one orone or more polishing treatment selected from the group consisting of(1) electrolytic polishing, (2) chemical polishing, and (3) mechanicalpolishing like wire brushing is applied to said outer side surfaceand/or said inner side surface of the pipe before said N absorption anddiffusion treatment and said stainless steel pipe and said adhesion pipematerial before adhesion processing of said <7> above.

<12>

The process for manufacturing a high nitrogen stainless steel pipeaccording to any one of <5> to <11> above, characterized in thatfurthermore heat treatment such as (1) annealing, (2) aging or (3)solution heat treatment is applied to said manufacturing processaccording to circumstances.

<13>

A product formed of the high nitrogen austenitic stainless steel pipematerial according to any one of <1> to <4> above, wherein said productis a high-pressure hydrogen gas container and a liquid hydrogencontainer which are for fuel cell vehicles (FCVs) or the like or astainless steal pipe or hollow material which can have large or smallsizes and be of various kinds.

According to the invention, the critical temperature for crystal grainenlargement of the stainless steel pipe material during the nitrogenabsorption-diffusion processing as mentioned above is judged to be atemperature between 1110 to 1120° C., i.e., a little exceeding 1100° C.,from the result the mean crystal grain diameter of the pipe materialprocessed at a temperature of 1120 to 1130° C. was rapidly increased to200 to 300 μm while that of the pipe material processed at a temperatureup to 1100° C. was near 100 μm.

According to the invention, as nitrogen absorption and diffusionprocessing is applied to an austenitic stainless steel pipe material, inthe case where the stainless steel pipe material is treated in nitrogengas atmosphere at a temperature of 1050 to 1100° C., in a range oftemperatures not higher than the critical temperature for crystal grainenlargement of the steel material, for hr to cause nitrogen (N) to beabsorbed into the surface of the steel material and diffused into thesolid phase, the resulting stainless steel material has a gradientstructure which comprises a part that is close to the steel surface partand has been highly strengthened by the formation of a highconcentration-solid solution of N and a part in which ductilitygradually increases toward around the center of the cross-section of thesteel as the N concentration decreases, and the enlargement of crystalgrains is minimized during the treatment. Thereafter, as the Nabsorption and diffusion processed stainless steel pipe material isannealed in argon gas at 1050 to 1100° C. for 24 hr, there is obtained anobel austenite stainless steel pipe material with high strength andductility and excellent corrosion and heat resistance that can never beachieved by conventional processes, and which is much more reinforced byslight plastic working such as drawing, rolling, extrusion or the like.

According to the invention, as grain refining processing utilizingeutectoid transformation of the austenite is applied to the austenitestainless steel pipe material having undergone nitrogenabsorption-diffusion and annealing treatments, there is obtained anextremely fine crystal grain structure.

Thus, the synergistic effects of the solid solution strengtheningoriginally coming from the gradient structure formed by nitrogen and theenhanced crystal grain refinement are combined with the ductility(elongation) inherent in the austenite phase to make it easy tomanufacture a highly strong and ductile austenite stainless steel pipematerial.

According to the invention, as adhesion processing such as hot drawingand hot rolling is applied to a plurality of austenite stainless steelpipes having undergone said treatments of nitrogen absorption-diffusion,annealing, and grain refining as stated above, it is formed into aunited austenite stainless steel pipe or hollow material with highstrength and ductility which has dimensions, e.g., diameter and wallthickness according to the use or strength level required and which hasrepetitions of the gradient structure, as described above, within thepipe wall, existence of such gradient structure making it easy topertinently adjust the mechanical properties thereof.

Thus obtained united stainless steel pipe having particularly largedimensions and high strength level cannot be manufactured by usingsingle such stainless steel pipe alone and by using conventionalprocesses too.

According to the invention, in high nitrogen austenite stainless steel,the characteristic of anti-HGE (anti-hydrogen gas embrittlement) seen inhigh nickel austenite stainless steel is still more enhanced by thesolid solution nitrogen present in said gradient structure, because thenitrogen especially promotes short range ordering (SRO) ofsubstitutional alloying elements leading to homogenization ofdistribution of alloying elements in the austenite phase throughincrease in concentration of free electron providing more metalliccharacter of interatomic bonds, such homogenization of distribution ofalloying elements in austenite phase also leading to excellent pittingcorrosion resistance as well as stabilization thereof.

According to the invention, as nitrogen absorption-diffusion processingis applied to an austenite stainless steel pipe material in nitrogen gasatmosphere, a gradient structure as stated above is formed within thepipe wall, and nitrogen (N) contained in the gradient structure, whose Nconcentration range is wide, lowers stacking fault energy (SFE) of theaustenite phase in pipe material, so that the extended dislocationcontaining stacking fault is stabilized to higher temperature side, thesoftening temperature rising around 100° C.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is illustrative of Vickers hardness (Hv) of cross-section ofaustenite stainless steel pipe of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %)having undergone the treatments, as used in one specific example of theinvention.

● (sample b): 30-hr holding in N₂ gas at 1075° C. (N absorption anddiffusion into solid phase)

◯ (sample c): 30-hr holding in N₂ gas at 1075° C.+24-hr holding in argongas at 1075° C. (Annealing)

⋄ (sample a): Commercial material.

FIG. 2 is illustrative of temperature dependence of tensile strength(σ_(B)) of austenite stainless steel pipe samples a and b ofFe-18Cr-12Ni-2.5Mo-0.06C (mass %), as used in one specific example ofthe invention.

◯ (sample b): 30-hr holding in N₂ gas at 1075° C.+24-hr holding in argongas at 1075° C.+grain refining

□ (sample a): Commercial material.

BEST MODE FOR CARRYING OUT THE INVENTION

Some embodiments of the invention are now explained. In one embodimentof the invention, nitrogen absorption and diffusion processing isapplied to an austenite stainless steel pipe. In this processing, theaustenite stainless steel pipe, surface-electropolished, (outside dia 26mm, 3 mm thick) of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) is treated innitrogen gas at a temperature of 1070 to 1100° C., which is not higherthan the critical temperature for crystal grain enlargement of the steelmaterial, for 30 hr to cause nitrogen (N) to be absorbed into thesurface of the steel pipe material and diffused into the solid phase,followed by annealing in argon gas at a temperature of 1070 to 1100° C.for 24 hr.

The resulting stainless steel pipe material has a gradient structurewhich comprises a part that is close to the steel surface part and hasbeen highly strengthened by the formation of a high concentration solidsolution of N and a part in which ductility gradually increases towardaround the center of the cross-section of the steel as the Nconcentration decreases, and the enlargement of crystal grain isminimized during the processing, thereby obtaining a high nitrogenaustenite stainless steel pipe material with high strength andductility.

Furthermore, if grain refining (i.e., crystal grain size reducing)treatment utilizing, for instance, eutectoid transformation of theaustenite is applied to the austenite stainless steel pipe materialhaving undergone both the nitrogen absorption-diffusion and theannealing processing, it is then possible to manufacture much moreimproved steel pipe material, because there is obtained an extremelyfine austenite with crystal grains of the order of 10 to 30 μm leadingto a marked elongation of the pipe material.

Synergistic effects of the solid solution strengthening originallycoming from the gradient structure formed by nitrogen and the enhancedcrystal grain refinement are combined with the ductility (elongation)inherent in the austenite phase to make it easy to manufacture a highlystrong and ductile austenite stainless steel pipe material.

Moreover, the resulting steel pipe material is more strengthened byplastic working such as drawing, rolling, extrusion or the like.

It is thus possible to achieve effective manufacture ofhigh-nitrogen-concentration austenite stainless steel pipe materialshaving the gradient structure as mentioned above.

In yet another embodiment of the invention, adhesion processing isapplied to a plurality of austenite stainless steel pipes with the samechemical composition but different dimensions having undergone saidtreatments of nitrogen absorption-diffusion, annealing, and grainrefining as described above using a hot drawing, hot rolling or othermethods at a temperature of 1050 to 1100° C. in H₂ gas atmosphere.

The resultant united austenite stainless steel pipe or hollow materialhas dimensions, e.g., diameter and wall thickness according to the useor strength level required and has repetitions of the gradientstructure, as mentioned above, within the pipe wall, existence of suchgradient structures in the united stainless steel material, making iteasy to adjust pertinently the mechanical properties thereof.

It is also thus possible to achieve more effective manufacture of theunited austenite stainless steel pipe or hollow material withparticularly large dimensions and high strength level that cannot bemanufactured by using single stainless steel pipe material alone.

EXAMPLES

Examples of the invention are now explained with reference to theaccompanying drawings.

Example 1

Set out in Table 1 are the mean crystal grain diameter D, offset yieldstrength σ_(0.2), tensile strength σ_(B), and elongation δ of stainlesssteel pipe samples (26 mm outside dia×150 mm long×3 mm thick) of (a)Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) and (b) Fe-20Cr-8Ni-0.03C (mass %)obtained by applying nitrogen (N) absorption and solid diffusiontreatment to their surface-electropolished samples in 0.1 MPa N₂ gas atboth the temperatures of 1075° C. and 1200° C. for 30 hr, followed byannealing in argon gas at the same temperatures as those of N absorptionand diffusion treatment for 24 hr.

TABLE 1 Mean crysral grain diameter D, offset yield strength σ_(0.2),tensile strength σ_(B), and elongation δ of stainless steal pipe samples(26 mm outside dia × 150 mm long × 3 mm thick) of (a)Fe—18Cr—12Ni—2.5Mo—0.02C (mass %) and (b) Fe—20Cr—8Ni—0.03C (mass %)obtained by applying nitrogen (N) absorption and solid diffusiontreatment to their surface-elec- tropolished samples in 0.1 MPa N₂ gasat both the temperatures of 1075° C. and 1200° C. for 30 hr, followed byannealing in argon gas at the same temperatures as those of N absorptionand diffusion treatment for 24. D σ_(0.2) σ_(B) Temperature ° C. μm MPaMPa δ % a 1075 115 510 850 50.0 1200 562 593 872 5.7 (Commercialmaterial) 100 265 549 68.0 b 1075 107 525 890 45.0 1200 493 624 908 4.6(Commercial material) 95 285 598 66.4 Value of D: microscopicallydetermined. Test pieces: as cut from tublar material/gauge length 50mm/gripped ends inserted with metal plugs.

From a comparison of the results obtained at 1075° C. of both the sample(a) and the sample (b) with those obtained at 1200° C., it has beenfound that according to the invention although the values of offsetyield strength σ_(0.2) and tensile strength σ_(B) of the samplesprocessed at 1075° C. and 1200° C. are remarkably increased, the valuesof elongation δ of these samples processed at 1200° C. are extremelydecreased.

Example 2

Set out in Table 2 are the mean crystal grain diameter D, offset yieldstrength σ_(0.2), tensile strength σ_(B), and elongation δ of stainlesssteel pipe sample (26 mm outside dia×150 mm long×3 mm thick) ofFe-18Cr-12Ni-2.5Mo-0.02C (mass %) obtained by applying nitrogenabsorption and solid diffusion treatment to the surface-electropolishedsample in 0.1 MPa NH₃ gas at a temperature of 520° C. for 60 h, followedby annealing in argon gas at a temperature of 1075° C. for 24 h.

TABLE 2 Mean crystal grain diameter D, offset yield strength σ_(0.2),tensile strength σ_(B), and elongation δ of stainless steel pipe sample(26 mm outside dia × 150 mm long × 3 mm thick) ofFe—18Cr—12Ni—2.5Mo—0.02C (mass %) obtained by applying nitrogenabsorption and solid diffusion treatment to the surface- electropolishedsample in 0.1 MPa NH₃ gas at a temperature of 520° C. for 60 h, followedby annealing in argon gas at a temperature of 1075° C. for 24 h. Dσ_(0.2) σ_(B) μm MPa MPa δ % N absorption · solid diffusion: 107 520 86540.3 520° C. × 60 hr in NH₃ gas annealing: 1075° C. × 24 hr in argon gas(Commercial material) 100 265 549 68.0 Value of D: microscopicallydetermined. Test pieces: as cut from tublar material/gauge length 50mm/gripped ends inserted with metal plugs.

Example 3

The stainless steel pipe sample surface-electropolished (26 mm outsidedia×150 mm long×3 mm thick) of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %), wasprocessed by 30-hr holding in N₂ gas at 1075° C. [Nabsorption-Diffusion] (sample a), followed by 24-hr annealing in argongas [Annealing] (sample b) and 20%* cold drawing [Drawing] (sample d)*[(D ₀ −D _(x))/D ₀]×100=20(%)

where D₀ and D_(x) are outside diameter of pipe before and after colddrawing, respectively.

The offset yield strength σ_(0.2), tensile strength σ_(B), elongation δ,and mean crystal grain diameter D microscopically determined are shownin table 3.

TABLE 3 The offset yield strength σ_(0.2), tensile strength σ_(B),elongation δ, and mean crystal grain diameter D of said sample a, sampleb, and sample d described in Example 3 of Fe—18Cr—12Ni—2.5Mo—0.02C (mass%) pipe material. a b c d σ_(0.2) MPa 502 510 265 769 σ_(B) MPa 685 850549 1016 δ % 28.3 50.0 68.0 42.0 c: commercial material test pieces: ascut from tublar material/gauge length 50 mm/gripped ends inserted withmetal plugs.

Example 4

FIG. 1 is illustrative of Vickers hardness (Hv) of cross-section ofaustenite stainless pipe (outside dia 26 mm×150 mm long×3 mm thick) ofFe-18Cr-12Ni-2.5Mo-0.02C (mass %) having undergone the treatments.

-   ● (sample b): 30-hr holding in N₂ gas at 1075° C. (N absorption and    diffusion into solid phase)-   ◯ (sample c): 30-hr holding in N₂ gas at 1075° C.+24-hr holding in    argon gas at 1075° C. (Annealing)-   ⋄ (sample a): Commercial material.    Hv value/N (mass %) of the surface part of sample (b) and (c) were    about 550/0.9 and 400/0.7, respectively.

From FIG. 1, it has been seen that although Vickers hardness Hv of thesurface part of the N absorption processed sample (b) is markedlyincreased due to the formation of solid solution of N compared to thecenter part remaining a little increase in Hv, in annealing processedsample (c), diffusion of N is promoted so that there appeared aconsiderable N solid solution hardening even in the center part, thegradient of Hv vs D curve becoming gentle.

It has been here noted that within the pipe wall in the sample, e.g. C,is formed a gradient structure which comprise a part that is close tothe surface part of the pipe and has been highly strengthened by theformation of a high-concentration solid solution of N and a part inwhich ductility gradually increases toward around the center of thecross-section of the pipe as the N concentration decreases.

Example 5

The stainless steel pipe sample surface-electropolished (outside dia 26mm×150 mm long×3 mm thick) of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) wasprocessed by 30-hr holding in N₂ gas at 1075° C. [N absorption andDiffusion] and 24-hr annealing in argon gas at 1075° C.[Annealing](sample a) followed by crystal grain refining (1) (sample b)and crystal grain refining (2) (sample c) described below.

The offset yield strength σ_(0.2), tensile strength σ_(B), elongation δ,and mean crystal grain diameter D are shown in Table 4. The values of Dwere microscopically determined.

TABLE 4 Mean crystal grain diameter D, offset yield strength σ_(0.2),tensile strength σ_(B), and elongation δ of said samples a, b, and cdescribed in explanation of Example 5 of Fe—18Cr—12Ni—2.5Mo—0.02C (mass%) pipe material. D σ_(0.2) σ_(B) Sample μm MPa MPa δ % a 115 510 85050.0 b 30 716 1004 65.0 c 34 692 973 59.7 Test pieces: as cut fromtublar material/gauge length 50 mm/gripped ends inserted with metalplugs.Crystal Grain Refining (Treatment) (1)

N absorption—Diffusion and Annealing processed said sample (a) was grainrefined by the following processes [1] to [7]:

[1] heating to 1200° C. (austenitizing)

[2] 5 to 6-minutes holding at 1200° C.

[3] air cooling (decomposition of austenite (γ) into fine ferrite (α)and nitride (Cr₂N))

[4] reheating to 1200° C. (formation fine of austenite (γ))

[5] 3 to 4-minutes holding at 1200° C.

[6] rapid cooling (water quenching) to room temperature

[7] fine austenite sample (b)

More fine austenite sample (b) is obtained by making use of 2repetitions of a series of the above procedures [1] to [7]

i.e., crystal grain refining treatment (1) is based on the eutectoidtransformation of austenite as shown in the following equation

Crystal Grain Refining (Treatment) (2)N absorption-Diffusion and Annealing processed said sample (a) wasgrain-refined by the following procedures [1] to [4]:

[1] 50% drawing* near 200 to 250° C., i.e., below recrystallizationtemperature, (work hardening)

[2] heating to 1200° C. (formation of fine austenite) and 3 to 4-minutesholding at the same temperature

[3] rapid cooling (water cooling)

[4] fine austenite sample (c)*[(D ₀ −D _(x))/D ₀]×100=50(%)

where D₀ and D_(x) are outside diameter of pipe before and afterdrawing, respectively.

The stainless steel pipe sample surface-electropolished (26 mm outsidedia×150 mm long×3 mm thick) of Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) wasprocessed by 30-hr holding in N₂ gas at 1130° C., which is in a range oftemperatures exceeding the critical temperature for crystal grainenlargement, and 24-hr annealing in argon gas at said temperature(sample a), followed by grain refining utilizing the grain refiningtreatment (1) stated in the example 5 (sample b).

The crystal grain diameter D microscopically determined, offset yieldstrength σ_(0.2), tensile strength σ_(B), and elongation δ are shown inTable 5.

TABLE 5 Mean crystal grain diameter D, offset yield strength σ_(0.2),tensile strength σ_(B), and elongation δ of said samples a and bdescribed in the present Example 6 of Fe—18Cr—12Ni—2.5Mo—0.02C (mass %)pipe material. D σ_(0.2) σ_(B) sample μm MPa MPa δ % a 250 569 860 15.0b 65 755 942 45.5 Test pieces: as cut from tublar material/gauge length50 mm/gripped ends inserted with metal plugs.

Example 7

The ferritic stainless steel pipe samples surface-electropolished(outside dia 26 mm×20 mm long×4.0 mm thick) of (a) Fe-18.0Cr-0.07C (mass%) pipe and (b) Fe-18.0Cr-0.06C-0.07Al (mass %) pipe whose cuter surfacewas shielded from ambient atmosphere by welding both edges of the pipecovered with 0.2 mm thick nickel pipe, were processed by 30-hr holdingin 0.65 MPa N₂ gas at 1075° C., followed by cooling in 0.65 MPa N₂ gasand 24-hr annealing in 0.1 MPa argon gas.

The mean crystal grain diameter D and the resulting structure of theouter surface part and the inner surface part are described in Table 6.

TABLE 6 Mean crystal grain diameter D and structure of outer surfacepart and inner surface part of ferritic stainless steel pipe samples ofthe dimension of outside diameter 26 mm, length 20 mm, and thickness 4.0mm of said pipe (a) Fe—18.0Cr—0.07C (mass %) and (b)Fe—18.0Cr—0.06C—0.07Al (mass %) processed by 30-hr holding in 0.65 MPaN₂ gas at 1075° C., followed by cooling in 0.65 MPa N₂ gas and 24-hrannealing in 0.1 MPa argon gas at 1075° C. D μm Al inner surface part*Outer surface part* sample mass % <structure> <structure> a 0.001 453550 <austenite> <austenite> b 0.070 112 133 <austenite> <ferrite>*Distance from surface: 0.20 mmThe mean crystal grain diameter of the patent samples of (a) and (b)without the treatment of N absorption-diffusion and Annealing were 100μm and 90 μm, respectively.

Example 8

FIG. 2 is illustrative of temperature dependence of tensile strength(σ_(B)) of austenite stainless steel pipe samples (outside dia 26 mm×150mm long×3 mm thick) (a) and (b) of Fe-18Cr-12Ni-2.5Mo-0.06C (mass %).

-   ◯ (sample b): 30-hr holding in N₂ gas at 1075° C.+24-hr holding in    argon gas at 1075° C.+grain refining-   □ (sample a): Commercial material.

From Example 8, FIG. 2, it has been found that the tensile strengthσ_(B) of the N absorption-diffusion and grain refining processed sample(b) is strikingly increased to compared to that of sample (a) in a widerange of temperature due to the synergistic effects of N solid solutionstrengthening and reduction of crystal grain.

It has also been found that according to the invention in the sample(b), nitrogen (N) contained in the gradient structure, whose Nconcentration range is wide, decreases stacking fault energy (SFE) ofthe austenite phase thereof, so that the extended dislocation containingstacking fault (SF) is stabilized to higher temperature side and thesoftening temperature rises about 100° C.

Example 9

The austenite stainless steel pipe samples surface electropolished (22mm outside dia×130 mm long×3 mm thick) of

(a) Fe-18Cr-12Ni-2.5Mo-0.02C (mass %) (SUS316L), (b) Fe-20Cr-8Ni-0.03C(mass %) (SUS304L), and (c) Fe-25Cr-20Ni-0.06C (mass %) (SUS310S) wereprocessed by 30-hr holding in N₂ gas at 1075° C. [N absorption andDiffusion], followed by 24-hr annealing in argon gas at 1075° C.[Annealing], then grain refining using grain refining treatment (1)described in the example 5, and finally 20%* drawing*[(D ₀ −D _(x))/D ₀]×100=20(%)where D₀=outside diameter of pipe before drawing

D_(x)=outside diameter of pipe after drawing

The offset yield strength σ_(0.2), tensile strength σ_(e), andelongation δ of the samples (a), (b), and (c) are shown in Table 7. Herethe samples (A), (B), and (C) are the parent materials (commercialmaterials) of the samples (a), (b), and (c), respectively.

TABLE 7 Offset yield strength σ_(0.2), tensile strength σ_(B), andelongation δ of said samples (a) and (A) of Fe—18Cr—12Ni—2.5Mo—0.02C(mass %), said samples (b) and (B) of Fe—20Cr—8Ni—0.03C (mass %), andsaid samples (c) and (C) of Fe—25Cr—20Ni—0.06C (mass %) described inexplanation of Example 9. σ_(0.2) σ_(B) sample MPa MPa δ % a 794 115050.0 A 265 549 68.0 b 850 1206 46.2 B 285 598 66.4 c 948 1300 37.0 C 312656 43.0 Test pieces: as cut from tublar material/gauge length 50mm/gripped ends inserted with metal plugs.

From Example 9, Table 7, it has been noted that according to theinvention, the values of offset yield strength δ_(0.2) and tensilestrength δ_(B) of the austenite stainless steel pipe samples, havingundergone the treatments of absorption-diffusion, annealing, grainrefining, and slight plastic working as mentioned in said explanation,are around 3 times and 2 times as high as those of commercial materials,respectively, in addition to the values of elongation δ comparedfavorably with those of commercial ones.

Example 10

Inner surface of pipe (sample (a), outer surface of pipe (sample (b)),and both surface of pipe (sample (c)) of stainless steel ofFe-18Cr-12Ni-3.5Mo-0.02C (mass %) were processed by 30-hr holding in N₂gas atmosphere at 1075° C., followed by 24-hr annealing in argon gasatmosphere at 1075° C., then crystal grain refining using grain refiningtreatment (2) described in Example 5, and finally 20%* cold drawing asshown in Example 9. Here atmosphere of outer surface side of sample (a)and inner surface side (i.e. in pipe) of example (b) were filled withargon gas.

The offset yield strength σ_(0.2), tensile strength σ_(B), andelongation δ are shown in Table 8.

TABLE 8 Offset yield strength σ_(0.2), tensile strength σ_(B), andelongation δ of said sample a, b, and, c described in explanation ofExample 10. σ_(0.2) σ_(B) sample MPa MPa δ % a 669 1004 49.0 b 690 102048.0 c 775 1095 46.3 Test pieces: as cut from tublar material/gaugelength 50 mm/gripped ends inserted with metal plugs.

Example 11

Set out in Table 9 are the offset yield strength σ_(0.2), tensilestrength σ_(B), and elongation δ of the stainless steel pipe sample (α),(α^(ND)), (β) and (θ) manufactured from a plurality of nitrogenabsorption-diffusion and annealing treated stainless steel pipes(Fe-18Cr-12Ni-3.5Mo-0.02C, mass %) using the following process (asstated in <7> above).

The processes of manufacture of said samples (α), (α^(ND)), (β) and (θ)are as follows.

Sample (α):

pipe sample A (37^(OD)×250^(L)×3t, mm, i.e., 37 mm in outside dia by 250mm long by 3 mm thick), pipe sample B (47^(OD)×250^(L)×3^(t), mm), andpipe sample C (57^(OD)×250^(L)×3^(t), mm) were treated by 30-hr holdingin N₂ gas at 1075° C. and 24-hr annealing in argon gas at 1075° C.,followed by practicing 2 repetitions of grain-refining treatment (1)described in Example 5 and disposing a plurality of said A, B, and C oneover another, then adhesion-processing through 15% hot drawing in H₂ gasat 1075° C. to unite said plurality of the pipe A, B, and C, and finally20% cold drawing to result in higher strength level.

[M2] The manufacturing process is summarized as follows.

Manufacturing process:

$\left. {N\mspace{14mu}{Absorption}\text{-}{Diffusion}}\rightarrow\left. \rightarrow\left. {Annealing}\rightarrow\left. {{Grain}\mspace{14mu}{refining}}\rightarrow\left. \rightarrow\begin{Bmatrix}{{{Disposing}\mspace{14mu}{one}\mspace{14mu}{over}\mspace{14mu}{another}} -} \\{{Adhesion}\mspace{14mu}{processing}} \\\left( {{Hot}\mspace{14mu}{drawing}} \right)\end{Bmatrix} \right. \right. \right. \right. \right.$$\left. {{Cold}\mspace{14mu}{drawing}\mspace{14mu}\ldots\mspace{14mu}{pipe}\mspace{14mu}{sample}\mspace{14mu}(\alpha)}\rightarrow{\begin{Bmatrix}{{Without}\mspace{14mu}{cold}} \\{drawing}\end{Bmatrix}\mspace{14mu}\ldots\mspace{14mu}{pipe}\mspace{14mu}{sample}\mspace{14mu}\left( \alpha^{ND} \right)} \right.$Sample (β):

Pipe sample A (37^(OD)×250^(L)×3^(t), mm), whose inner surface wasshielded from ambient atmosphere by welding both edges of the pipecovered with 0.2 mm thick Ni pipe, pipe sample B (47^(OD)×250^(L)×3^(t),mm), and pipe sample C (57^(OD)×250^(L)×3^(t), mm),surface-electropolished were treated by 30-hr holding in N₂ gas at 1075°C. and 24-hr annealing in argon gas at 1075° C., followed by disposing aplurality of said samples A, B, and C one over another, thenadhesion-processing through 15% hot drawing in H₂ gas at 1075° C. tounite the plurality of said pipes A, B, and C, and finally 20% colddrawing to result in higher strength level.

[M3]

Manufacturing Process:

$\left. {N\mspace{14mu}{Absorption}\text{-}{{Diffusion}\text{}\begin{pmatrix}{{{all}\mspace{14mu}{the}\mspace{14mu}{surfaces}\mspace{14mu}{of}\mspace{14mu}{samples}\mspace{14mu} A},B,{{and}\mspace{14mu} C},} \\\left. {{except}\mspace{14mu}{for}\mspace{14mu}{inner}\mspace{14mu}{surface}\mspace{14mu}{of}\mspace{14mu}{sample}\mspace{14mu} A} \right)\end{pmatrix}}}\rightarrow\left. \rightarrow\left. {Annealing}\rightarrow\left. \begin{Bmatrix}{{{Disposing}\mspace{14mu}{one}\mspace{14mu}{over}\mspace{14mu}{another}} -} \\{{Adhesion}\mspace{14mu}{processing}} \\\left( {{Hot}\mspace{14mu}{drawing}} \right)\end{Bmatrix}\rightarrow\left. \rightarrow{{Cold}\mspace{14mu}{drawing}\mspace{14mu}\ldots\mspace{14mu}{pipe}\mspace{14mu}{sample}\mspace{14mu}(\beta)} \right. \right. \right. \right. \right.$Sample (θ):

Each of 2 austenite stainless steel pipes, surfaced-polished, with 0.7mm thickness as adhesion materials (having the name quality with SUS316Lstainless steel pipe) without N absorption-Diffusion and Annealingtreatment was sandwiched between pipe samples A and B, and between pipesamples B and C, respectively.

Thus obtained adhesion material-sandwiched steel pipe were treated as inthe case of the sample (a)

[M4]

Manufacturing Process:

$\left. {N\mspace{14mu}{a{bsorption}}\text{-}{Diffusion}}\rightarrow\left. {Annealing}\rightarrow\left. {{Grain}\mspace{14mu}{refining}}\rightarrow\text{}\left. \begin{Bmatrix}{{Adhesion}\mspace{14mu}{material}\mspace{14mu}{sandwiching}} \\\left( {{Between}\mspace{14mu}{A/B}\mspace{14mu}{and}\mspace{14mu}{B/C}} \right)\end{Bmatrix}\rightarrow\left. \begin{Bmatrix}{{{Disposing}\mspace{14mu}{one}\mspace{14mu}{over}\mspace{14mu}{another}} -} \\{{Adhesion}\mspace{14mu}{processing}} \\\left( {{Hot}\mspace{14mu}{drawing}} \right)\end{Bmatrix}\rightarrow{{Cold}\mspace{14mu}{drawing}\mspace{14mu}\ldots\mspace{14mu}{pipe}\mspace{14mu}{sample}\mspace{14mu}(\theta)} \right. \right. \right. \right. \right.$

TABLE 9 Offset yield strength σ_(0.2), tensile strength σ_(B), andelongation δ of said sample α, α^(ND) β, and θ described in the presentExample 11 of Fe—18Cr—12Ni—3.5Mo—0.02C (mass %) pipe material sample αα^(ND) β θ σ_(0.2) MPa 787 720 725 760 σ_(B) MPa 1110 995 945 1077 δ %52.4 62.0 43.0 55.0 Test pieces: as cut from tublar material/gaugelength 50 mm/gripped ends inserted with metal plugs.

From Example 11, Table 9, it has been found that according to theinvention, the values of offset yield strength σ_(0.2) of each of thesamples α, α^(ND), β, and θ, which are basically formed from Nabsorption-diffusion and annealing processed steel pipe materials, ofFe-18Cr-12Ni-3.5Mo-0.02C (mass %) are about 3 times as high as that ofthe commercial pipe material in addition to the values of elongation ofthese samples compared favorably with that of the commercial one.

Furthermore, it has been noted that the elongation of the sample α^(ND)without final cold drawing processing is as good as the commercial pipematerial in spite of the high strength similar to those of other samplesα, β and θ.

Mechanics of materials teaches that when fluid like gas of pressure (P)is filled in cylindrical vessel with the inside dia (D) and the wallthickness (t), the maximum stress acting along the circumferencedirection ((e) so as to tear up the vessel is given by the followingequation, in the case where the value of t is within 10% lower than thatof D.σ_(θ)=(PD)/2t  [M5].According to eq [M5], since the parameters P, D, and t are kept inequilibrium with one another, when the values of P and D are constant,the value of thickness of the cylindrical vessel t decreases in inverseproportion to the value of σ_(e), i.e., the strength level of the vesselitself; the required thickness t of the vessel decreases to around ⅓compared to that of the vessel manufactured from conventional pipematerial.

POSSIBLE APPLICATIONS OF THE INVENTION TO THE INDUSTRY

The high-nitrogen austenite stainless steel pipe and hollow materialsformed therefrom obtained herein are now explained with reference towhat purposes they are used for.

High-Nitrogen Austenite Stainless Steel Pipe

High-nitrogen austenite steel pipe materials have common properties asstated below. They have high strength and ductility, and show excellentpitting corrosion resistance, crevice corrosion resistance andnon-magnetism as well. Furthermore, they do not undergo sharp softeningfrom the temperature of near 200 to 300° C. on temperature rises, whichis usually experienced with steel materials of the martensite or ferritetype, and they are less susceptible to low-temperature brittleness at atemperature at or lower than room temperature.

Another important feature of noteworthiness is that one exemplaryhigh-nitrogen austenite stainless steel pipe material of SUS 316L typeof the invention which has undergone the nitrogen (N)absorption-diffusion and the grain refining processing has an offsetyield strength about three times as high as that of the SUS 316Lstainless steel pipe material, in addition to the value of elongationcompared favorably with those of commercial materials.

According to the invention as the N absorption-diffusion processing isapplied to austenite stainless steel pipe material, there is obtained agradient structure whose N concentration range is wide. The N containedin the gradient structure decreases stacking fault energy (SFE) of theaustenite phase in the pipe, so that the extended dislocation containingstacking fault is stabilized to enhance the strength level as well asheat resistance thereof.

Furthermore, as adhesion processing is applied to a plurality ofaustenite stainless steel pipes which have undergone the Nabsorption-diffusion and the grain refining processing, a unitedaustenite stainless steel pipe with dimensions according to the use orstrength level required can easily be produced, which cannot be realizedto produce by using single such austenite pipe alone.

In addition, in high nitrogen austenite stainless steel of theinvention, characteristic of anti-HGE (anti-hydrogen gas embrittlement)that can be seen in high nickel austenite stainless steel such as SUS316L or SUS 310S austenite stainless steel unlike many other metallicmaterials is still more enhanced by the solid solution nitrogen,contained in said gradient structure, that promotes short range ordering(SRO) leading to homogenization of distribution of alloying elements inthe austenite phase through increase in concentration of free electronproviding more metallic character of interatomic bonds.

Thus, the high-nitrogen austenite stainless steel pipe materials of theinvention, because of having such features as mentioned above, cansuitably find a wide spectrum of applications inclusive of high strengthand ductility materials as follows:

Compressed hydrogen gas storage tank materials for fuel cell vehicle(FCV), Pipe arrangement materials for steam power generation,petrochemistry etc. and Sea related machinery and tools materials.

What is claimed is:
 1. A high nitrogen stainless steel pipe materialwith ductility, and a corrosion and heat resistance, comprising: anaustenite steel having a chemical composition comprising Cr, Ni, Mo, C,and Fe, containing 0.25 to 1.7% (by mass) of solid solution nitrogenincluding a nitrogen concentration gradient structure throughout, andincluding an outside surface and/or an inside surface of a pipe in whicha concentration of solid solution nitrogen continuously decreasesgradually from the outside and/or inside surface, wherein said austenitesteel including said gradient structure comprises a part that is theoutside and/or inside surface and has a high concentration solidsolution of nitrogen and a part in which ductility gradually increasestoward around a center of a cross-section of the pipe as the nitrogenconcentration decreases, and an extended dislocation generated in anaustenite phase by the solid solution nitrogen present in said gradientstructure is stabilized to enhance yield strength level of the austenitephase and ductility thereof so that the solid solution nitrogencontained in said gradient structure also still more enhancescharacteristic of anti-hydrogen gas embrittlement, that can be seen inhigh nickel austenite stainless steel, as a strong austenite stabilizerelement of nitrogen itself.
 2. The high nitrogen stainless steel pipematerial with ductility, and a corrosion and heat resistance accordingto claim 1, wherein said pipe material comprises a kind of steelselected from the group consisting of austenitic stainless steel,ferritic stainless steel, and ferrite-austenite stainless steel.
 3. Ahigh nitrogen stainless steel pipe material with ductility, and acorrosion and heat resistance, comprising: the pipe according to claim1, the pipe comprising a plurality of pipe members wherein the pipemembers are disposed one over another and metallurgically bonded toobtain interfaces between nitrided surfaces of the pipe members andsurfaces not undergoing nitriding of the pipe members leading to aunited high nitrogen austenite steel pipe material.
 4. A product formedof the high nitrogen austenitic stainless steel pipe material havinghigh yield strength, leading to weight reducing and the ductility, andenhancing anti-hydrogen gas embrittlement through stabilization of theaustenite phase ascribed to solid solution nitrogen present in saidnitrogen-concentration gradient structure, according to claim 1, whereinsaid product is a high-pressure hydrogen gas container and a liquidhydrogen container which are for fuel cell vehicles or a stainless steelpipe or a hollow material.
 5. A high nitrogen stainless steel pipematerial with ductility, and a corrosion and heat resistance,comprising: an austenite steel having a chemical composition comprisingCr, Ni, Mo, C, and Fe (iron), and containing 0.25 to 1.7% (by mass) ofsolid solution nitrogen including a nitrogen-concentration gradientstructure throughout including a surface layer, within a pipe wall inwhich a concentration of solid solution nitrogen continuously decreasesgradually from a surface, wherein said high nitrogen stainless steel hasaustenite grains of an order of a few tens of μm refined through aeutectoid transformation of an austenite or refined through arecrystallization of a work-hardened austenite, and wherein asynergistic effect of the solid solution nitrogen present in saidgradient structure combined with crystal grains refined leads toaustenite stainless steel pipe materials having a high yield strengthand ductility originated from the refined and enhanced crystal grainsincluded in said gradient structure, and extended dislocation containingstacking faults is stabilized to enhance yield strength as well as heatresistance due to the extended dislocation generated in an austenitephase.
 6. The high nitrogen stainless steel pipe material withductility, and a corrosion and a heat resistance according to claim 5,wherein said pipe material comprises a kind of steel selected from thegroup consisting of austenitic stainless steel, ferritic stainlesssteel, and ferrite-austenite stainless steel.
 7. A high nitrogenstainless steel pipe material with ductility, and a corrosion and heatresistance, comprising: the pipe consisting of all austenite stainlesssteel pipe according to claim 5, the pipe comprising a plurality of pipemembers, wherein the pipe members are disposed one over another andmetallurgically bonded to obtain interfaces between nitrided surfaces ofthe pipe members and surfaces not undergoing nitriding of the pipemembers leading to a united high nitrogen austenite stainless steel pipematerial having a predetermined thickness, and having a high yieldstrength and ductility, with a multi-gradient structure.
 8. A productformed of the high nitrogen austenitic stainless steel pipe materialhaving high yield strength leading to weight reducing and ductility, andenhancing anti-hydrogen gas embrittlement through stabilization of theaustenite phase ascribed to solid solution nitrogen present in saidnitrogen-concentration gradient structure, according to claim 5, whereinsaid product is a high-pressure hydrogen gas container and a liquidhydrogen container which are for fuel cell vehicles or a stainless steelpipe or a hollow material.